JP2007147811A - Fixing device and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fixing device which easily and surely prevents an excessive temperature rise by self-temperature control of a heating member without slowing down the rise of the device and has the heating member having a simple structure and is free from troubles such as break of the heating member and a fixing member even in continuous feeding of small-sized paper, and an image forming apparatus. <P>SOLUTION: The fixing device includes; a magnetic field generation means 25 for generating an alternating magnetic field; a heating member 23 which is disposed so as to have front and rear sides spaced from and interposed between the magnetic field generation means 25, and generates heat by the alternating magnetic field; and a varying means which varies distances M1 and M2 from at least one of the front and rear sides of the heating member 23 to a magnetic field generation means 25a by positions in a breadthwise direction. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electromagnetic induction heating type fixing device and an image forming apparatus such as a copying machine, a printer, a facsimile, or a complex machine including the same.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer, a technique using an electromagnetic induction heating type fixing device is widely known for the purpose of reducing the rise time of the apparatus and saving energy (for example, patents). Reference 1).

  In Patent Document 1, etc., an electromagnetic induction heating type fixing device includes a support roller (heating roller), a fixing auxiliary roller (fixing roller), a fixing belt stretched between the supporting roller and the fixing auxiliary roller, and a fixing belt on the supporting roller. A magnetic field generating means (induction heating means) opposed to each other via a fixing roller, a pressure roller contacting the fixing auxiliary roller via a fixing belt, and the like. The magnetic field generating means includes an excitation coil extending in the width direction (a direction perpendicular to the recording medium conveyance direction), an excitation coil core facing the excitation coil, and the like.

The fixing belt is heated at a position facing the magnetic field generating means. The heated fixing belt heats and fixes the toner image on the recording medium conveyed to the positions of the auxiliary fixing roller and the pressure roller. Specifically, by passing a high-frequency alternating current through the exciting coil of the magnetic field generating means, an alternating magnetic field is formed around the exciting coil, and an eddy current is generated in the vicinity of the surface of the support roller. When an eddy current is generated in the support roller, Joule heat is generated by the electrical resistance of the support roller itself. The fixing belt wound around the support roller is heated by the Joule heat.
Such an electromagnetic induction heating type fixing device raises the surface temperature (fixing temperature) of the fixing belt to a desired temperature in a short start-up time with less energy consumption as compared with other types such as a heat roller type. Known to be warm.

  On the other hand, Patent Document 2 and the like disclose a fixing device that uses an electromagnetic induction heating method, and uses a magnetic conductor having a Curie point for a heat generating member (heat generating portion) to give the heat generating member a self-temperature control function. It is disclosed as prior art.

  Further, Patent Document 2 discloses a fixing device that uses an electromagnetic induction heating method, and even when the heat generating member is provided with a self-temperature control function using a magnetic conductor having a Curie point, In order to prevent an increase in temperature rise time (rise time) at the time of raising, a magnetic field generating means (excitation means) using a laminate of two magnetic metal members having different Curie points as a heat generating member A technique for varying the frequency of the alternating current supplied to the power supply is disclosed.

JP-A-2005-70376 Japanese Patent No. 3399849

  Since the fixing device disclosed in Patent Document 2 and the like described above has a self-temperature control function for the heat generating member, it performs complex temperature control by an electric circuit as compared with that of Patent Document 1 and the like. And overheating of the fixing member can be prevented. However, various problems also occurred.

  Specifically, in the vicinity of the Curie point of the heat generating member, the relative permeability of the heat generating member (conductive layer) is lowered, and the temperature rising gradient of the heat generating member is reduced. Therefore, there is a high possibility that the temperature rising time at the start-up of the fixing device will be excessive for the region where the temperature rising gradient becomes small. That is, although the excessive temperature rise can be prevented by the self-temperature control of the heat generating member, the start-up of the fixing device is insufficient.

  On the other hand, by using a laminate of two magnetic metal members having different Curie points as the heat generating member, and changing the frequency of the alternating current supplied to the magnetic field generating means, the temperature rises due to self-temperature control of the heat generating member. The effect of preventing the temperature and shortening the rise of the fixing device can be expected. That is, by adjusting the frequency of the alternating current, the magnetic metal member having a Curie point exceeding the fixing temperature is heated at the start-up of the apparatus to prevent the temperature rise time from decreasing, and the fixing temperature after the apparatus starts up A magnetic metal member having a Curie point in the vicinity is heated to prevent overheating of the heating member.

  However, although such a fixing device can be expected to achieve both shortening of the rise time and prevention of excessive temperature rise, the heat generating member has a multilayer structure and the structure becomes complicated and expensive. There was a bug. Furthermore, since the heat generating member has a multi-layered structure with two magnetic metal members having different linear expansion coefficients, there is a high possibility that the heat generating member will be thermally strained due to the difference in linear expansion coefficient. When heat distortion occurs in the heat generating member in this way, the layers of the heat generating member are peeled off and damaged, or the performance of the heat generating member (for example, belt conveyance performance when the heat generating member is a fixing belt). ) Will be reduced.

  Further, when the above-described fixing device continuously passes a recording medium (small size paper) having a small size in the width direction, the temperature of the sheet passing area in the fixing member is decreased and the temperature is quickly increased. Therefore, a magnetic metal member having a high Curie point is caused to generate heat. Therefore, the temperature of the non-sheet passing region in the fixing member may be excessively raised in a short time, and the fixing member may be thermally damaged.

  The present invention has been made to solve the above-described problems, and prevents overheating easily and reliably by self-temperature control of the heat generating member without slowing down the start-up of the apparatus, and It is an object of the present invention to provide a fixing device and an image forming apparatus that have a simple structure and do not cause problems such as breakage of a heat generating member or a fixing member even when small-size paper is continuously passed.

The inventor of the present application has come to know the following matters as a result of repeated studies to solve the above-mentioned problems.
That is, when a material having a Curie point (a material capable of self-temperature control) is used for the heat generating member in order to prevent overheating of the heat generating member, the magnetic field generating means for generating an alternating magnetic field is provided on the heat generating member. When the magnetic field generating means is disposed so as to sandwich the front and back surfaces of the heat generating member (the heat generating main surface and the opposite surface) as compared with the case where the heat generating main surface is disposed. This increases the ability of self-temperature control in the heat generating member. Moreover, even if the heat generating member has a single layer structure, the alternating current of the heat generating member can be changed by changing the facing position (facing distance) of the magnetic field generating means arranged so as to sandwich the front and back surfaces of the heat generating member. The temperature rise stop temperature (the temperature at which the temperature rise gradient becomes almost zero) can be changed without changing the frequency. Furthermore, by varying the facing distance of the magnetic field generating means to the heat generating member according to the position in the width direction, the temperature distribution in the width direction of the heat generating member can be optimized even when small-size paper is continuously passed.

  The present invention is based on the above-described matters. That is, the fixing device according to the first aspect of the present invention is a fixing device for fixing a toner image to a recording medium, and generates a magnetic field that generates an alternating magnetic field. And a heat generating member that generates heat by the alternating magnetic field, and at least one of the front and back surfaces of the heat generating member. Variable means for changing the facing distance of the magnetic field generating means according to the position in the width direction.

  The fixing device according to a second aspect of the present invention is the fixing device according to the first aspect, wherein the variable means has a range in which the facing distance is within the range according to the size of the recording medium in the width direction. It is controlled to be shorter than the facing distance outside.

  According to a third aspect of the present invention, in the fixing device according to the first or second aspect, the variable means changes from a small size in the width direction of the recording medium to a large size. Then, the facing distance within the range in the width direction of the small one is controlled to be longer than the facing distance outside the range in the width direction of the small one.

  The fixing device according to a fourth aspect of the present invention is the fixing device according to any one of the first to third aspects, wherein the variable means has a short facing distance over the entire width direction when the apparatus is started up. It is controlled to become.

  According to a fifth aspect of the present invention, in the fixing device according to any one of the first to fourth aspects, the variable means is a heat generating main surface of the front and back surfaces of the heat generating member. Only the facing distance to the surface can be varied depending on the position in the width direction.

  The fixing device according to a sixth aspect of the present invention is the fixing device according to any one of the first to fifth aspects, wherein the variable means deforms the magnetic field generating means linearly or / and curvedly. It is something to be made.

  A fixing device according to a seventh aspect of the present invention is the fixing device according to any one of the first to sixth aspects, wherein the magnetic field generating means is disposed once or a plurality of times on the front and back surfaces of the heat generating member. The coil is wound so as to be sandwiched between them.

  The fixing device according to an eighth aspect of the present invention is the fixing device according to any one of the first to seventh aspects, wherein the heat generating member is a conductive member formed to have a Curie point of 350 ° C. or lower. With a layer.

  According to a ninth aspect of the present invention, there is provided the fixing device according to any one of the first to eighth aspects, wherein the alternating current supplied to the magnetic field generating means for generating the alternating magnetic field is generated. The frequency is in the range of 10 k to 1 MHz.

  A fixing device according to a tenth aspect of the present invention is the fixing device according to any one of the first to ninth aspects, wherein the heat generating member is a fixing member that melts a toner image.

  The fixing device according to an eleventh aspect of the present invention is the fixing device according to the tenth aspect, wherein the fixing member is a fixing roller that abuts against a pressure roller that presses a recording medium to be conveyed. The magnetic field generating means is disposed so as to face the outer peripheral surface and the inner peripheral surface of the fixing roller.

  The fixing device according to a twelfth aspect of the present invention is the fixing device according to the tenth aspect of the present invention, wherein the fixing member is a fixing belt stretched in a circumferential shape, and the magnetic field generating means is the fixing device. The belt is disposed so as to face the outer peripheral surface and the inner peripheral surface of the belt.

  According to a thirteenth aspect of the present invention, in the fixing device according to the twelfth aspect, the fixing belt is stretched between a support roller and a fixing auxiliary roller, and the fixing auxiliary roller is conveyed. It is disposed so as to abut against a pressure roller for pressing the recording medium via the fixing belt.

  According to a fifteenth aspect of the present invention, in the fixing device according to the thirteenth aspect, the magnetic field generating means is disposed so as to face the inner peripheral surface of the fixing belt via the support roller. It has been done.

  According to a fifteenth aspect of the present invention, in the fixing device according to any one of the first to ninth aspects, the heating member is a heating member that heats the fixing member that melts the toner image. Is.

  The fixing device according to a sixteenth aspect of the present invention is the fixing device according to the fifteenth aspect, wherein the fixing member is a fixing belt, and the heating member stretches the fixing belt together with a fixing auxiliary roller. And the magnetic field generating means is disposed so as to face the outer peripheral surface of the fixing belt and to face the inner peripheral surface of the fixing belt via the support roller. Is arranged so as to come into contact with a pressure roller that presses the conveyed recording medium via the fixing belt.

  According to a seventeenth aspect of the present invention, an image forming apparatus includes the fixing device according to any one of the first to sixteenth aspects.

  In the present invention, the magnetic field generating means is disposed so as to sandwich the front and back surfaces of the heat generating member, and the opposing distance of the magnetic field generating means to the heat generating member is variable depending on the position in the width direction. The self-temperature control performance is improved, the heating member has a single layer structure, and the temperature rise stop temperature can be changed without changing the frequency of the alternating current, and the temperature distribution in the width direction of the heating member is optimized Can be As a result, overheating is easily and reliably prevented by self-temperature control of the heat generating member without slowing the start-up of the device, and the structure of the heat generating member is simple, even when small-size paper is continuously passed. It is possible to provide a fixing device and an image forming apparatus that do not cause problems such as breakage of the heat generating member and the fixing member.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings. In addition, in each figure, the same code | symbol is attached | subjected to the part which is the same or it corresponds, The duplication description is simplified or abbreviate | omitted suitably.

Embodiment 1 FIG.
The first embodiment of the present invention will be described in detail with reference to FIGS.
First, the configuration and operation of the entire image forming apparatus will be described with reference to FIG.
In FIG. 1, 1 is a main body of a laser printer as an image forming apparatus, 3 is an exposure unit that irradiates a photosensitive drum 18 with exposure light L based on image information, and 4 is detachably installed on the apparatus main body 1. A process cartridge as an image forming unit; 7, a transfer unit for transferring a toner image formed on the photosensitive drum 18 to a recording medium P; 10, a paper discharge tray on which an output image is placed; A paper feeding unit containing a recording medium P such as paper, 13 a registration roller for conveying the recording medium P to the transfer unit 7, 15 a manual paper feeding unit, 18 a photosensitive drum as an image carrier, and 20 a recording An electromagnetic induction heating type fixing device for fixing an unfixed image on a medium P is shown.

With reference to FIG. 1, an operation during normal image formation in the image forming apparatus will be described.
First, exposure light L such as laser light based on image information is emitted from the exposure unit 3 toward the photosensitive drum 18 of the process cartridge 4. The photosensitive drum 18 rotates counterclockwise in the drawing, and a toner image corresponding to image information is formed on the photosensitive drum 18 through a predetermined electrophotographic process (charging process, exposure process, development process). Is done. Thereafter, the toner image formed on the photosensitive drum 18 is transferred onto the recording medium P conveyed by the registration roller 13 in the transfer unit 7.
Although not shown, the process cartridge 4 contains a photosensitive drum 18, a charging unit that charges the photosensitive drum 18, and toner (developer), and is formed on the photosensitive drum 18. A developing unit that develops the electrostatic latent image, a cleaning unit that removes untransferred toner remaining on the photosensitive drum 18, and the like are integrally provided.

On the other hand, the recording medium P conveyed to the transfer unit 7 operates as follows.
First, one of the plurality of paper feeding units 11, 12, and 15 of the image forming apparatus main body 1 is automatically or manually selected (for example, the uppermost paper feeding unit 11 is selected). To do.) Then, the uppermost sheet of the recording medium P stored in the paper feeding unit 11 is transported toward the position of the transport path K. Thereafter, the recording medium P passes through the conveyance path K and reaches the position of the registration roller 13. Then, the recording medium P that has reached the position of the registration roller 13 is conveyed toward the transfer unit 7 at the same timing in order to align with the toner image formed on the photosensitive drum 18.

After the transfer process, the recording medium P passes through the position of the transfer unit 7 and then reaches the fixing device 20 through the conveyance path. The recording medium P that has reached the fixing device 20 is fed between the fixing belt and the pressure roller, and the toner image is fixed by the heat received from the fixing belt and the pressure received from the pressure roller. The recording medium P on which the toner image has been fixed is sent from between the fixing belt and the pressure roller, and then is discharged from the image forming apparatus main body 1 as an output image and placed on the paper discharge tray 10.
Thus, a series of image forming processes is completed.

Next, the configuration and operation of the fixing device 20 in the image forming apparatus main body 1 will be described in detail with reference to FIG.
As shown in FIG. 2, the fixing device 20 mainly includes a fixing auxiliary roller 21, a fixing belt 22, a support roller 23, an induction heating unit 24, a pressure roller 30, a thermistor 38, a guide plate 35, a separation plate 36, and the like. Is done.

  Here, the fixing auxiliary roller 21 is formed by forming an elastic layer such as silicone rubber on the surface of a cored bar made of stainless steel, carbon steel or the like. The elastic layer of the fixing auxiliary roller 21 has a thickness of 3 to 10 mm and an Asker hardness of 10 to 50 degrees. The fixing auxiliary roller 21 is rotationally driven counterclockwise in FIG. 2 by a driving device (not shown).

  The support roller 23 as a heating member (heating member) includes a conductive layer (cylindrical portion) made of a magnetic conductive material. The cylindrical portion of the support roller 23 is formed so that its thickness (layer thickness) is about 0.6 mm. The support roller 23 rotates counterclockwise in FIG. A coil 25 is disposed so as to face the front and back surfaces (the outer peripheral surface and the inner peripheral surface) of the support roller 23 (see FIG. 4).

Here, as a material of the support roller 23 as a heat generating member, a magnetic conductive material such as nickel, iron, chromium, or an alloy thereof can be used. In the first embodiment, the support roller 23 is formed of only a conductive layer and has a single layer structure. Specifically, as the material (conductive layer) of the support roller 23, the Curie point is not less than the target value of the control temperature of the fixing belt 22 (the fixing target temperature is about 150 ° C.) and not more than 350 ° C. A magnetic shunt alloy that falls within the above range is used. Specifically, it is an alloy of nickel, iron, and chromium, and a desired Curie point can be obtained by adjusting the amount of each material added and the processing conditions. Thus, by forming the support roller 23 with the conductive layer having the Curie point near the fixing target temperature of the fixing belt 22, the support roller 23 is heated without being excessively heated by electromagnetic induction. . This will be described in detail later.
In the first embodiment, the support roller 23 has only a conductive layer. However, a reinforcing layer, an elastic layer, a heat insulating layer, or the like may be provided on the conductive layer of the support roller 23.

Hereinafter, the fixing belt 22 will be described in detail.
Referring to FIG. 2, a fixing belt 22 (fixing member) as a heat generating member is stretched and supported by a support roller 23 and a fixing auxiliary roller 21.
As shown in FIG. 3A, the fixing belt 22 is a multi-layered endless belt in which a conductive layer 22b, an elastic layer 22c, and a release layer 22d are sequentially formed on a base material 22a. The base material 22a is made of an insulating heat-resistant resin material, and for example, polyimide, polyamideimide, PEEK, PES, PPS, fluorine resin, or the like can be used. The layer thickness of the base material 22a is formed to be 30 to 200 μm from the viewpoint of heat capacity and strength.

The conductive layer 22b (heat generation layer) of the fixing belt 22 is made of a magnetic conductive material and has a layer thickness of 1 to 20 μm. The conductive layer 22b is formed on the base material 22a by plating, sputtering, vacuum deposition or the like.
Here, a magnetic conductive material such as nickel or stainless steel can be used as the material of the conductive layer 22b. In the first embodiment, a magnetic shunt alloy having a Curie point higher than the fixing target temperature and lower than 350 ° C. is used as the material of the conductive layer 22b. Specifically, it is an alloy of nickel, iron, and chromium, and a desired Curie point can be obtained by adjusting the amount of each material added and the processing conditions. Thus, by forming the conductive layer 22b with a magnetic conductive material having a Curie point near the fixing temperature of the fixing belt 22, the conductive layer 22b is heated without being excessively heated by electromagnetic induction. Become. This will be described in detail later.

  The elastic layer 22c of the fixing belt 22 is made of silicone rubber, fluorosilicone rubber, or the like, and is formed to have a layer thickness of 50 to 500 μm and an Asker hardness of 5 to 50 degrees. Thereby, a uniform image quality without gloss unevenness can be obtained in the output image.

The release layer 22d of the fixing belt 22 is made of tetrafluoroethylene resin (PTFE), tetrafluoroethylene / perfluoroalkyl vinyl ether copolymer resin (PFA), tetrafluoroethylene / hexafluoropropylene copolymer (FEP). ), A mixture of these resins, or a dispersion of these resins in a heat resistant resin. The layer thickness of the release layer 22d is 5 to 50 μm (preferably 10 to 30 μm). Thereby, the toner releasability on the fixing belt 22 is ensured and the flexibility of the fixing belt 22 is ensured.
A primer layer or the like may be provided between the layers 22 a to 22 d of the fixing belt 22.

In the first embodiment, the fixing belt 22 as the heat generating member is a four-layer structure (the structure shown in FIG. 3A), but the multilayer structure shown in FIGS. 3B to 3D is used. It can also be a body.
3B includes a conductive layer 22b, an elastic layer 22c, and a release layer 22d. Here, as the conductive layer 22b, a material obtained by dispersing magnetic conductive particles in a resin material such as polyimide, polyamideimide, PEEK, PES, PPS, or a fluororesin can be used. In that case, magnetic conductive particles are added within a range of 20 to 98% by weight with respect to the resin material. Specifically, the magnetic conductive particles are dispersed in a resin material in a varnish state using a dispersing device such as a roll mill, a sand mill, or a centrifugal defoaming device. This is adjusted to an appropriate viscosity with a solvent and formed into a desired layer thickness with a mold.

In the fixing belt 22 of FIG. 3C, a plurality of conductive layers 22b are provided in a base material 22a, and an elastic layer 22c and a release layer 22d are sequentially formed thereon.
In the fixing belt 22 of FIG. 3D, an elastic layer 22c including a plurality of conductive layers 22b is formed on a base material 22a, and a release layer 22d is further formed as a surface layer.
Also in these cases, the same effect as in the first embodiment can be obtained by configuring the conductive layer 22b in the same manner as in the first embodiment.

2 and 4, the induction heating unit 24 as a magnetic field generating means for generating an alternating magnetic field is constituted by a coil 25 formed in a loop shape.
Here, the coil 25 as the magnetic field generating means is wound apart so as to sandwich the front and back surfaces (the inner peripheral surface and the outer peripheral surface) of the fixing belt 22 and the support roller 23 as the heat generating member once. Excitation coil. In other words, the fixing belt 22 and a part of the support roller 23 are sandwiched in the loop of the loop-shaped coil 25. As shown in FIG. 4, the coil 25 extends in parallel to the width direction of the fixing belt 22 and the support roller 23. One end of the coil 25 in the width direction is a folded portion that connects the inner peripheral surface side and the outer peripheral surface (the heat generation main surface) side, and the other end is connected to the high-frequency power supply unit 40. Then, an alternating current of 10 k to 1 MHz (preferably 20 k to 300 kHz) is applied to the coil 25 from the high frequency power supply unit 40.

Here, the coil 25 is a litz wire obtained by bundling a plurality of thin conductive wires having an insulating coating on the surface. In general, the smaller the diameter of the conducting wire, the smaller the loss when a high-frequency alternating current is applied, but the strength is reduced and breakage due to winding is likely to occur. Therefore, the coil preferably has a wire diameter of each conductive wire of 0.05 mm or more. Moreover, it is preferable that the strand diameter of each conducting wire is not more than twice the penetration depth calculated from the frequency of the alternating current applied to the coil in consideration of current flowing through the entire strand. The penetration depth δ is obtained by the following equation.
δ = 503 · [ρ / (μf)] ^1/2
In the above equation, ρ is the volume resistivity of the material, μ is the relative permeability of the material, and f is the frequency of the alternating current that excites the material.

  In addition, when the coil is a litz wire, if the number of twists is large, the cross-sectional area increases, so the current resistance increases, but the flexibility for winding decreases, and the occupied area increases and the layout disadvantages It becomes. Considering these matters, the coil 25 of the first embodiment uses a twisted wire of 150 conductors having a strand diameter of 0.15 mm.

In the first embodiment, the coil 25 is wound around the front and back surfaces of the fixing belt 22 and the support roller 23 so as to be sandwiched only once (the number of turns is 1), but is shown in FIG. As described above, the coil 25 can be wound around the front and back surfaces of the fixing belt 22 and the support roller 23 so as to be spaced apart a plurality of times. At this time, the number of turns of the coil 25 is preferably 1 to 50 times, and more preferably 1 to 10 times.
In the first embodiment, the coil 25 is composed of a litz wire, but the coil 25 can also be composed of a single conducting wire.
Further, in order to prevent the leakage magnetic field from being formed in the region where the coil 25 does not face the front and back surfaces of the support roller 23 and the fixing belt 22, a core is provided to shape the magnetic path, or copper, aluminum, or the like is used. It is also possible to install a nonmagnetic low resistance conductor cover.

The induction heating unit 24 (coil 25) as the magnetic field generation unit configured as described above is in the direction of the broken line arrow (in the radial direction of the support roller 23) by the drive unit 51 as the variable unit with reference to FIG. It is configured to be movable.
Specifically, referring to FIGS. 6 and 7, in the coil 25, the opposing distance to the front surface (Otemen) among the front and back surfaces of the heat generating members 22 and 23 is variable depending on the position in the width direction. It is configured to be able to. In the first embodiment, since only the facing distance of the coil 25 to the surfaces (outer peripheral surfaces) of the heat generating members 22 and 23 is varied depending on the position in the width direction, the configuration of the drive unit 51 becomes relatively simple. 6 and 7, reference numeral 26 denotes a bearing that rotatably supports the support roller 23 in the fixing device.

The facing distance of the coil 25a facing the surface of the heat generating member is varied in the range of M1 to M2 depending on the position in the width direction. Specifically, the facing distance of the coil 25a is varied depending on the position in the width direction in accordance with the paper passing conditions such as the size in the width direction of the recording medium and the state of the apparatus. The control of the facing distance of the coil 25 will be described in detail later.
In the first embodiment, the facing distance of the coil 25b facing the back surface of the heat generating member is fixed at a predetermined value N (see FIG. 9), but the facing distance of the coil 25b on the back surface side is the width. It can also be configured to vary depending on the position in the direction.

  Referring to FIG. 2, the pressure roller 30 is formed by forming an elastic layer such as fluororubber or silicone rubber on a cylindrical member made of aluminum, copper or the like. The elastic layer of the pressure roller 30 has a thickness of 1 to 5 mm and an Asker hardness of 20 to 50 degrees. The pressure roller 30 is in pressure contact with the auxiliary fixing roller 21 via the fixing belt 22. Then, the recording medium P is conveyed to a contact portion (a fixing nip portion) between the fixing belt 22 and the pressure roller 30.

A guide plate 35 for guiding the conveyance of the recording medium P is disposed on the entrance side of the contact portion between the fixing belt 22 and the pressure roller 30.
A separation plate 36 that guides the conveyance of the recording medium P and promotes the separation of the recording medium P from the fixing belt 22 is disposed on the exit side of the contact portion between the fixing belt 22 and the pressure roller 30. Yes.

On the outer peripheral surface of the fixing belt 22 and on the upstream side of the fixing nip portion, a thermistor 38 (temperature sensing element) having high thermal response is in contact. The thermistor 38 detects the surface temperature (fixing temperature) on the fixing belt 22 and adjusts the output of the induction heating unit 24.
A plurality of thermistors 38 may be installed in the width direction, and the facing distance of the coil 25a facing the surface of the heat generating member may be varied depending on the position in the width direction based on the detection results of the plurality of thermistors.

The fixing device 20 configured as described above operates as follows.
By the rotation driving of the auxiliary fixing roller 21, the fixing belt 22 rotates in the direction of the arrow in FIG. 2, the support roller 23 also rotates counterclockwise, and the pressure roller 30 also rotates in the direction of the arrow. The fixing belt 22 is heated at a position facing the coil 25 (the position of the support roller 23).

  Specifically, the high frequency alternating current of 10 kHz to 1 MHz is caused to flow from the high frequency power supply unit 40 to the coil 25 so that the magnetic field lines are alternately switched in both directions in the loop of the coil 25. By forming the alternating magnetic field in this way, when the temperature of the support roller 23 and the conductive layer 22b is equal to or lower than the Curie point, an eddy current is generated on the surface of the support roller 23 and the conductive layer 22b of the fixing belt 22, Joule heat is generated by the electrical resistance of the support roller 23 and the conductive layer 22b, and the support roller 23 and the conductive layer 22b are heated. Thus, the fixing belt 22 is heated by the heat received from the heat-generating support roller 23 and the heat generation of its own conductive layer 22b.

Thereafter, the surface of the fixing belt 22 that generates heat by the coil 25 passes through the position of the thermistor 38 and reaches a contact portion with the pressure roller 30. Then, the toner image T on the conveyed recording medium P is heated and melted.
Specifically, the recording medium P carrying the toner image T through the image forming process described above is fed between the fixing belt 22 and the pressure roller 30 while being guided by the guide plate 35 (indicated by the arrow Y). It is movement in the transport direction.) The toner image T is fixed to the recording medium P by the heat received from the fixing belt 22 and the pressure received from the pressure roller 30, and the recording medium P is sent from between the fixing belt 22 and the pressure roller 30.

  The surface of the fixing belt 22 that has passed through the position of the pressure roller 30 then reaches the position facing the coil 25 again. Such a series of operations is continuously repeated to complete the fixing step in the image forming process.

In such a fixing step, when the temperature of the support roller 23 and the conductive layer 22b exceeds the Curie point, heat generation of the support roller 23 and the conductive layer 22b is limited.
That is, when the temperature of the support roller 23 and the conductive layer 22b heated by the induction heating unit 24 exceeds the Curie point, the support roller 23 and the conductive layer 22b lose their magnetism, so that eddy currents near the surface Occurrence is limited. Thereby, the generation amount of Joule heat in the support roller 23 and the conductive layer 22b is reduced, and excessive temperature rise is suppressed.

  Such self-temperature control capability is obtained when the coil 25 is arranged in a loop shape with respect to the heat generating members 22b and 23 as in the first embodiment, and the heat generating main surface side (outer peripheral surface side) of the heat generating members 22b and 23. This is particularly high as compared with the case where the coil 25 is disposed so as to face only. Experimental examples showing such effects will be described later with reference to FIGS.

Here, a plurality of driving units 51 serving as variable means for changing the surface side coil 25a according to the position in the width direction are arranged in parallel in the width direction and push the coil 25a in the radial direction of the support roller 23 at each position. It is comprised with the pushing member. And the pushing member corresponding to the position which wants the coil 25a to adjoin to the support roller 23 is driven suitably.
Specifically, in the state of FIG. 6A, since none of the pushing members of the driving unit 51 is driven, the facing distance between the coil 25a and the support roller 23 is M2 over the entire width direction. Yes. Then, by driving the pushing member at the central portion in the width direction of the drive unit 51 from the state of FIG. 6A, the central portion in the width direction of the coil 25a is deformed in the direction of the broken line arrow, and the central portion in the width direction ( The facing distance in the range L1 is changed to M1 (the state shown in FIG. 6B). Note that by releasing the pushing by the pushing member at the center in the width direction from the state of FIG. 6B, the coil 25a is again brought into the state of FIG. 6A by the restoring force (elastic force) of the coil 25a. Become.

Such adjustment control of the facing distance based on the position of the coil 25a in the width direction is performed according to the sheet passing condition of the recording medium, the state of the apparatus, and the like.
Specifically, referring to FIG. 6B, the facing distance M1 within the range L1 is shorter than the facing distance M2 outside the range according to the size (size) of the recording medium P in the width direction. Is controlling. That is, when the recording medium P having the width direction size L1 is passed (particularly when continuous paper is passed), the facing distance at the center portion L1 in the width direction of the coil 25a is set to M1. As a result, in the range L1 where a large amount of heat is consumed in the sheet passing range, the coil 25a is close to the heat generating member (the support roller 23 and the fixing belt 22), and the heat generation efficiency of the heat generating member is improved. . On the other hand, in the non-sheet-passing range where the amount of heat is unnecessary (outside the range of L1), the coil 25a is not close to the heat generating member (the support roller 23 and the fixing belt 22). The heat generation efficiency of the member is reduced. In this manner, the temperature distribution in the width direction of the heat generating members 22 and 23 can be optimized in accordance with the size in the width direction of the recording medium (paper passing condition). Such control is particularly effective when small-size paper is continuously fed.

  In the first embodiment, the driving unit as the variable means deforms the coil 25a so that the opposing distance gradually increases linearly in a range L2 from the center portion to both ends of the coil 25a. On the other hand, as shown in FIG. 8A, the coil 25a can be deformed into a crank shape so that the opposing distance outside the range of the central portion L1 of the coil 25a is uniform. Further, as shown in FIG. 8B, the coil 25a can be deformed so that the facing distance gradually increases in a curve in a range L2 extending from the center of the coil 25a to both ends. In this way, a desired temperature distribution in the heat generating members 22 and 23 can be obtained by arbitrarily deforming the coil 25a linearly or / and curvilinearly.

  Further, in the first embodiment, referring to FIG. 7, when the size in the width direction of the recording medium P is changed from a small one to a large one (change from a small size paper to a large size paper). ), The facing distance M2 in the range L1 in the width direction of a small one (small size paper) is controlled to be longer than the facing distance M1 in the outside L2 in the width direction of the small one. That is, when the size in the width direction of the recording medium P to be passed is changed from (L1) to (L1 + L2 × 2), the facing distance at the central portion L1 of the coil 25a is set to M2, and outside the range L2 The facing distance at is set to M1. As a result, the coil 25a is in a range L2 (a range from a non-sheet passing range to a sheet passing range due to large-size sheet feeding) that is a non-sheet feeding range at the time of small-size sheet feeding and has reduced heat generation efficiency. Can be brought close to the heating members 22 and 23 to raise the temperature in a short time. On the other hand, in the range L1 in which sufficient heat generation efficiency is achieved even when a small size is passed, the heat generation efficiency is lowered without bringing the coil 25a close to the heat generation members 22 and 23. This balances with the heat generation efficiency in the out-of-range L2. In this manner, the temperature distribution in the width direction of the heat generating members 22 and 23 can be optimized in accordance with the change in the size of the recording medium in the width direction (paper passing condition).

  In the first embodiment, referring to FIG. 9, when the fixing device (image forming apparatus) is started up, the facing distance is controlled to be shortened over the entire width direction of the heat generating members 22 and 23. That is, when the temperature rise range is increased, as shown in FIG. 9A, the coil 25a is brought close to the heat generating members 22 and 23 so that the opposing distance is M1 in the entire width direction. On the other hand, when the fixing temperature is stable, the temperature increase range is relatively small, as shown in FIG. 9B, the opposing distance is set to M2 in the entire width direction of the coil 25a so as not to be close to the heat generating members 22 and 23. Thereby, according to the state of an apparatus, the heat_generation | fever efficiency of the width direction whole region in the heat generating members 22 and 23 can be optimized.

This will be described in more detail below.
The heating members 22 and 23 have a single-layer structure by changing the facing positions (facing distances) of the coils 25 arranged so as to sandwich the front and back surfaces of the heating members 22 and 23 with respect to the heating members 22 and 23. Even without changing the frequency of the alternating current, the temperature increase stop temperature (the temperature at which the temperature increase gradient becomes substantially zero) of the heat generating members 22 and 23 can be changed. Specifically, when the coil 25 is at the position shown in FIG. 9A, the magnetic flux passing through the heat generating members 22 and 23 is increased compared to when the coil 25 is located at the position shown in FIG. The temperature rise stop temperature increases.

Therefore, when the temperature on the surface of the fixing belt 22 is low and it is desired to quickly raise the temperature, the heat generation members 22 and 23 are overheated beyond the fixing target temperature by setting the coil 25 to the position shown in FIG. 9A. The temperature is raised in a short time. In particular, when the image forming apparatus 1 (fixing apparatus 20) is started up after the image forming apparatus 1 has been left for a long time, the amount of heat taken by members around the fixing apparatus 20 as the fixing apparatus 20 starts up is increased. In order to increase, the above-mentioned control becomes effective.
On the other hand, when the temperature of the surface of the fixing belt 22 has reached the vicinity of the fixing target temperature and it is not necessary to quickly raise the temperature, the heating members 22 and 23 are moved by setting the coil 25 to the position shown in FIG. Self-temperature control is performed in the vicinity of the temperature increase stop temperature (the temperature increase stop temperature decreased from the temperature increase stop temperature in FIG. 9A). Therefore, the position of the coil 25 in FIG. 9B is set so that the temperature rise stop temperature coincides with the temperature at which excessive temperature rise is prevented.
An experimental example showing the above-described effect will be described later with reference to FIG.

  As described above, the coil 25 (magnetic field generating means) is disposed so as to sandwich the front and back surfaces of the heat generating members 22 and 23, and the opposing distance of the coil 25a with respect to the heat generating members 22 and 23 is set to the position in the width direction. Therefore, even if the heating member (conductive layer) has a single layer structure and the frequency of the alternating current is not changed, the temperature rise stop temperature is improved. And the temperature distribution in the width direction of the heat generating members 22 and 23 can be optimized. As a result, the temperature rise of the heat generating members 22 and 23 can be prevented easily and reliably by the self-temperature control of the heat generating members 22 and 23 without slowing down the apparatus, and the structure of the heat generating members 22 and 23 is simple and small-size paper is continuously used. Even when the paper is passed, it is possible to prevent problems such as breakage of the heat generating member and the fixing member.

In the first embodiment, the fixing belt 22 having the conductive layer 22b and the support roller 23 having the conductive layer are used as the heat generating members. On the other hand, only one of the fixing belt 22 and the support roller 23 can be used as the heat generating member. Also in this case, a conductive layer similar to that of the first embodiment is provided on one member used as the heat generating member, the heat generating member is sandwiched between the loop-shaped coils 25, and the opposing distance of the coil 25 is set in the width direction. By varying the position, the same effect as in the first embodiment can be obtained.
In particular, when only the support roller 23 is a heat generating member, the conductive layer 22b of the fixing belt 22 is not necessary, and the support roller 23 has a single-layer structure (a structure having only a conductive layer). The overall configuration of the fixing device 20 is further simplified.

Embodiment 2. FIG.
A second embodiment of the present invention will be described in detail with reference to FIG.
FIG. 10 is a cross-sectional view illustrating a main part of the image forming apparatus according to the second embodiment. The image forming apparatus of the second embodiment is different from that of the first embodiment in that the image forming apparatus is a tandem color image forming apparatus and the fixing roller 31 is used as a heat generating member.

  The image forming apparatus according to the second embodiment is a tandem color image forming apparatus. As shown in FIG. 10, a plurality of photosensitive drums 18BK, 18Y, 18M, and 18C are arranged in parallel on the transfer belt 8 in the image forming unit. Although not shown, a charging unit, an exposure unit, a developing unit, a cleaning unit, and a charge eliminating unit are disposed on the outer periphery of the plurality of photosensitive drums 18BK, 18Y, 18M, and 18C (process cartridge 4 in FIG. 1). Can be referred to.) Then, a toner image of each color (black, yellow, magenta, cyan) is formed on each photosensitive drum 18BK, 18Y, 18M, 18C.

The transfer unit 7 includes a transfer belt 8 that transports the recording medium P, a bias roller 9 that faces each of the photosensitive drums 18BK, 18Y, 18M, and 18C via the transfer belt 8, and a cleaning roller that cleans the surface of the transfer belt 8. 14, etc.
The transfer belt 8 sequentially conveys the recording medium P conveyed from the direction of the arrow to a position facing each of the photosensitive drums 18Y, 18M, 18C, and 18BK. At this time, the toner images of the respective colors are transferred onto the recording medium P by the transfer bias applied to the bias roller 9. Thus, a full-color toner image is formed on the recording medium P. Thereafter, the recording medium P on which a full-color toner image is formed is separated from the transfer belt 8 and conveyed toward the fixing device 20.

On the other hand, the fixing device 20 of the second embodiment mainly includes a fixing roller 31 (fixing member) as a heat generating member, a pressure roller 30, an induction heating unit 24, and the like.
The fixing roller 31 includes a conductive layer 22b made of a magnetic conductive material, an elastic layer made of silicone rubber, a release layer made of a fluorine compound, or the like. As in the first embodiment, the conductive layer 22b of the fixing roller 31 is formed of a magnetic shunt alloy having a Curie point that is higher than the target fixing temperature and lower than 350 ° C. The fixing roller 31 has a mechanical strength that resists the pressure applied by the pressure roller 30.

Moreover, the induction heating part 24 is comprised with the coil 25 formed in the loop shape similarly to the said Embodiment 1. FIG. That is, the coil 25 is spaced apart so as to sandwich the front and back surfaces (the inner peripheral surface and the outer peripheral surface) of the fixing roller 31.
When an alternating current of 10 k to 1 MHz is supplied to the coil 25, an alternating magnetic field is generated in the loop of the coil 25, and the fixing roller 31 is heated by electromagnetic induction. In this manner, the electromagnetic induction heated fixing roller 31 heats and melts the toner image on the recording medium P conveyed from the direction of the arrow and fixes the toner image on the recording medium P.

  Further, in the fixing device according to the second embodiment, as in the first embodiment, the induction heating unit 24 (the coil 25 on the outer peripheral surface side) is broken by the broken line arrow by the driving unit as the variable means. In the direction (the radial direction of the fixing roller 31). Specifically, the facing distance of the coil 25 facing the surface of the fixing roller 31 (heat generating member) is varied depending on the position in the width direction according to the sheet passing condition and the state of the apparatus.

  As described above, according to the configuration of the second embodiment, the self-temperature control performance of the fixing roller 31 can be improved, the temperature rise stop temperature can be changed, and the temperature distribution in the width direction of the fixing roller 31 can be changed. Can be optimized. Accordingly, the temperature rise of the fixing roller 31 can be easily and reliably prevented without slowing the start-up of the apparatus, the structure of the fixing roller 31 is simple, and the heat generating member can be used even when small-size paper is continuously fed. It is possible to prevent problems such as damage to the fixing member and the like.

Embodiment 3 FIG.
A third embodiment of the present invention will be described in detail with reference to FIG.
FIG. 11 is a cross-sectional view showing the fixing device according to the third embodiment, and corresponds to FIG. 2 of the first embodiment. In the fixing device of the third embodiment, the position of the induction heating unit 24 is different from that of the first embodiment.

The fixing device 20 according to the third embodiment mainly includes a fixing belt 22 (fixing member), a heating roller 28 (heating member) as a heating member, an induction heating unit 24, a support roller 23, a pressure roller 30, and the like. The
As with the support roller 23 in the first embodiment, the heating roller 28 is formed of a magnetic shunt alloy having a Curie point that is higher than the fixing target temperature and lower than 350 ° C. The heating roller 28 is on the upstream side of the fixing nip (upstream in the traveling direction of the fixing belt 22), and is in contact with the inner peripheral surface of the fixing belt 22 with a predetermined pressure.

  As shown in FIG. 11, the induction heating unit 24 according to the third embodiment has a loop shape disposed at positions facing the outer peripheral surface and the inner peripheral surface of the fixing belt 22 from the support roller 23 to the auxiliary fixing roller 21. Coil 25. That is, the coil 25 is spaced apart so as to sandwich the front and back surfaces of the fixing belt 22 and the heating roller 28.

  In the fixing device 20 configured as described above, an alternating current of 10 k to 1 MHz is supplied to the loop-shaped coil 25, thereby generating an alternating magnetic field between the fixing belt 22 and the coil 25 sandwiching the heating roller 28. The heating roller 28 is heated by electromagnetic induction. In the third embodiment, the fixing belt 22 does not include a conductive layer, and receives heat from the heating roller 28 and reaches a desired fixing temperature.

  Further, in the fixing device according to the third embodiment, similarly to the first embodiment, the induction heating unit 24 (the coil 25 on the outer peripheral surface side) is indicated by the broken line arrow by the driving unit as the variable means. Moved in the direction. Specifically, the facing distance of the coil 25 facing the surface of the heating roller 28 (heat generating member) is varied depending on the position in the width direction according to the sheet passing condition and the state of the apparatus.

  As described above, according to the configuration of the third embodiment, the self-temperature control performance of the heating roller 28 can be improved, the temperature rise stop temperature can be changed, and the temperature distribution in the width direction of the heating roller 28 can be changed. Can be optimized. Accordingly, the heating roller 28 can be easily and reliably prevented from overheating without slowing down the start-up of the apparatus, and the heating roller 28 has a simple structure. It is possible to prevent problems such as damage to the fixing member and the like.

Embodiment 4 FIG.
A fourth embodiment of the present invention will be described in detail with reference to FIG.
FIG. 12 is a cross-sectional view showing the fixing device according to the fourth embodiment. The fixing device according to the fourth embodiment uses a cylindrical fixing belt 22 as a fixing member facing the induction heating unit 24, and uses a fixing roller 31 as a fixing member facing the induction heating unit 24. This is different from that of the second embodiment.

  The fixing device 20 according to the fourth embodiment mainly forms a fixing belt 22 (fixing member) as a heat generating member, a holding member 55 provided inside the fixing belt 22 to hold the fixing belt 22, and a desired fixing nip portion. For this purpose, the fixing belt 22 includes an elastic member 56, an induction heating unit 24 as a magnetic field generating means, a pressure roller 30, and the like. The fixing belt 22 includes a conductive layer having a desired Curie point, like the fixing belt 22 in the first embodiment.

Moreover, the induction heating part 24 is comprised with the coil 25 formed in the loop shape similarly to the said Embodiment 1. FIG. In other words, the coil 25 is spaced apart so as to sandwich the front and back surfaces of the fixing belt 22.
Then, when an alternating current of 10 k to 1 MHz is supplied to the coil 25, an alternating magnetic field is generated in the loop of the coil 25, and the fixing belt 22 is heated by electromagnetic induction. In this manner, the electromagnetic induction heated fixing belt 22 heats and melts the toner image on the recording medium P conveyed from the direction of the arrow and fixes the toner image on the recording medium P.

  Further, in the fixing device according to the fourth embodiment, similarly to the first embodiment, the induction heating unit 24 (the coil 25 on the outer peripheral surface side) is broken by the broken line arrow by the driving unit as the variable means. Moved in the direction. Specifically, the facing distance of the coil 25 facing the surface of the fixing belt 22 (heat generating member) is varied depending on the position in the width direction according to the sheet passing condition and the state of the apparatus.

  As described above, according to the configuration of the fourth embodiment, the self-temperature control performance of the fixing belt 22 can be improved to change the temperature rise stop temperature, and the temperature distribution in the width direction of the fixing belt 22. Can be optimized. Thus, the temperature rise of the fixing belt 22 can be easily and reliably prevented without slowing down the start-up of the apparatus, and the structure of the fixing belt 22 is simple, and even when small-size paper is continuously passed, the heating member It is possible to prevent problems such as damage to the fixing member and the like.

Experimental example.
An experimental example for confirming the effects described in the above embodiments will be described with reference to FIGS.
First, an experimental example will be described with reference to FIGS. 13 to 15 for confirming the effect of increasing the self-temperature control capability of the heat generating member by arranging the magnetic field generating means so as to sandwich the front and back surfaces of the heat generating member.
FIG. 13A and FIG. 13B are schematic views showing an experimental apparatus. In the experimental apparatus of FIG. 13A, the coil 25 is separated so as to sandwich the front and back surfaces of a test piece having a conductive layer 33 (corresponding to the heat generating member in each of the above embodiments). (This is the configuration of the fixing device in each of the embodiments.) In the experimental apparatus of FIG. 13B, the coil 25 is opposed to the main heat generating surface of the test piece having the conductive layer 33 (the structure of a conventional fixing apparatus).

  That is, the experimental apparatus in FIG. 13A and the experimental apparatus in FIG. 13B have the same configuration of the coil 25, and only the direction of the test piece facing the coil 25 is different.

  Here, the test piece includes only the conductive layer 33, a nonmagnetic conductive layer 34 made of aluminum on the conductive layer 33 with a thickness of 0.3 mm, and a non-conductive test piece made of aluminum on the conductive layer 33. Three types were prepared: a magnetic conductive layer 34 formed with a thickness of 0.8 mm. The conductive layer 33 of the test piece is made of a magnetic shunt alloy having a front and back size of 25 mm × 50 mm, a thickness of 0.22 mm, and a Curie temperature of 240 ° C. The nonmagnetic conductive layer 34 of the test piece also has a front and back size of 25 mm × 50 mm.

  Further, two types of alternating currents having an electric power of 200 to 1200 W and an excitation frequency of 36 kHz and 130 kHz are applied to the coil 25 of the experimental apparatus from the high frequency power supply unit 40. As a result, magnetic lines of force as shown in FIGS. 13A and 13B are formed in the vicinity of the coil 25.

14 and 15 are graphs showing the results of experimental examples performed using the above-described experimental apparatus. 14 and 15, the horizontal axis indicates the time since the start of electromagnetic induction, and the vertical axis indicates the temperature on the conductive layer 33.
FIG. 14 shows the experimental results when using the experimental apparatus of FIG. 13A, and FIG. 15 shows the experimental results when using the experimental apparatus of FIG. 13B.

  FIG. 14A is a graph showing the relationship between time and temperature when the frequency of the alternating current output from the high frequency power supply unit 40 is 36 kHz. FIG. 14B is a graph showing the relationship between time and temperature when the frequency of the alternating current output from the high frequency power supply unit 40 is 130 kHz. In FIG. 14, a solid line R0 is a case where a test piece having only the conductive layer 33 is used, and a solid line R1 is a test piece in which a nonmagnetic conductive layer 34 having a thickness of 0.3 mm is formed on the conductive layer 33. The solid line R2 is a case where a test piece in which a nonmagnetic conductive layer 34 having a thickness of 0.8 mm is formed on the conductive layer 33 is used.

  FIG. 15A is a graph showing the relationship between time and temperature when the frequency of the alternating current output from the high frequency power supply unit 40 is 36 kHz. FIG. 15B is a graph showing the relationship between time and temperature when the frequency of the alternating current output from the high frequency power supply unit 40 is 130 kHz. In FIG. 15, a solid line Q0 is a case where a test piece having only the conductive layer 33 is used, and a solid line Q1 is a test piece in which a nonmagnetic conductive layer 34 having a thickness of 0.3 mm is formed on the conductive layer 33. The solid line Q2 represents a case where a test piece in which a nonmagnetic conductive layer 34 having a thickness of 0.8 mm is formed on the conductive layer 33 is used.

From FIG. 14, it can be seen that, regardless of the presence or absence of the nonmagnetic conductive layer 34 and the frequency of the alternating current, when the temperature of the conductive layer 33 reaches the Curie point, further overheating is prevented.
On the other hand, FIG. 15A shows that when the excitation frequency is set to 36 kHz, overheating of the conductive layer 33 cannot be prevented unless the nonmagnetic conductive layer 34 having a thickness of 0.8 mm or more is provided. . Similarly, FIG. 15B shows that when the excitation frequency is set to 130 kHz, excessive heating of the conductive layer 33 cannot be prevented unless the nonmagnetic conductive layer 34 having a thickness of 0.3 mm or more is provided. Thus, when the coil 25 is opposed to the heat generating main surface of the heat generating member (conductive layer 33), it is necessary to provide a non-magnetic conductive layer having a low resistivity on the opposite side of the heat generating main surface. This is consistent with the contents described in Japanese Patent Application Laid-Open No. 2003-215956.

  From the above, it can be seen that the ability of the heat generating member to control the self-temperature can be enhanced by inserting the heat generating member into the loop-shaped coil 25. Furthermore, from comparison between FIG. 14 and FIG. 15, it can be seen that the heat generation efficiency (rise) of the heat generation member is improved by inserting the heat generation member in the loop-shaped coil 25. In addition, since the above-described effect can be obtained without providing the nonmagnetic conductive layer 34 on the heat generating member, the structure of the heat generating member is simplified. Therefore, it is possible to provide a heat generating member that is inexpensive and has no problems due to the provision of a conductive layer such as delamination.

Next, in FIG. 16, an experiment for confirming the effect of changing the temperature rise stop temperature in the heat generating member by changing the opposing position of the magnetic field generating means arranged so as to sandwich the front and back surfaces of the heat generating member. An example will be described.
In this experiment, the coil 25 is separated from the front and back surfaces of a heat generating member (a cylinder having an outer diameter of 20 mm, a thickness of 0.3 mm, and a width direction length of 50 mm) made of only a conductive layer having a Curie point of 190 ° C. Using the arranged device (corresponding to the fixing device of the first embodiment), the temperature rise characteristic when the facing distance of the coil 25 to the heat generating member is varied is measured. The frequency of the alternating current supplied to the coil 25 was 20 kHz, and the supplied power was 630 W (90 V × 7 A). Moreover, the opposing distance of the coil 25 with respect to the heat generating member was varied within a range in which M1 to M2 shown in FIGS. 9A and 9B were 1 to 5 mm.

  FIG. 16 is a graph showing the results of an experimental example performed using the experimental apparatus described above. In FIG. 16, the horizontal axis indicates the time from the start of electromagnetic induction, and the vertical axis indicates the temperature on the surface of the heat generating member (heat generating main surface). In FIG. 16, the solid line S1 indicates the temperature rise characteristic when the coil is at the position shown in FIG. 9A (M1 = 1 mm), and the solid line S3 is the coil at the position shown in FIG. 9B. (M2 = 5 mm). A solid line S2 indicates the temperature rise characteristic when the coil is in the middle position between the two (M2 = 3 mm).

  From the experimental results shown in FIG. 16, it was confirmed that the temperature rise stop temperature in the heat generating member changes by changing the opposing position of the coils that are spaced apart so as to sandwich the front and back surfaces of the heat generating member. Specifically, when the surface side coil is close to the heat generating member (in the case of the solid line S1), the temperature rise stop temperature is increased (about 230 ° C.), and the surface side coil is retracted from the heat generating member. In the case of the solid line S3, the temperature increase stop temperature decreases (about 150 ° C.).

  Utilizing such characteristics, as described in the above embodiments, rapid heating of the heating member (shortening of the startup time) and prevention of overheating by self-temperature control of the heating member, , And can be made compatible easily.

  Next, in FIG. 17, when small-size paper is continuously passed, the opposing distance of the magnetic field generating means arranged so as to sandwich the front and back surfaces of the heat generating member is variable according to the width of the small-size paper. Thus, an experimental example for confirming the effect of optimizing the temperature distribution in the width direction of the heat generating member will be described.

  In this experiment, using a device (corresponding to the fixing device of the first embodiment) in which the coils 25 are spaced apart on the front and back surfaces of the heat generating member consisting only of a conductive layer having a Curie point of 250 ° C. The temperature rise characteristic when the facing distance of the coil 25 with respect to is varied is measured. The temperature is adjusted by the thermistor 38 so that the temperature at the center in the width direction of the fixing belt 22 is 160 ° C.

  In FIG. 17, the horizontal axis indicates the time from the start of paper passing, and the vertical axis indicates the temperature on the fixing belt 22 during paper passing. In FIG. 17, the solid line W1 indicates the end portion of the fixing belt (the temperature rise width is equal to M2 when the coil is in the state of FIG. 6A or FIG. 9B) (the opposing distance in the entire width direction is M2). The solid line W2 indicates the temperature rise characteristic at the fixing belt end when the coil is in the state shown in FIG. 9A (the opposing distance in the entire width direction is M1). A solid line W3 indicates the temperature rise characteristic of the end portion of the fixing belt when the coil is in the state shown in FIG. 6B (the facing distance at the central portion in the width direction is M1). A solid line W4 indicates the temperature rise characteristic of the fixing belt central portion (the temperature is adjusted to be 160 ° C. in the sheet passing region).

From the solid line W1 and the solid line W2, when the opposing distance of the coil 25 to the heat generating member is shortened in the entire width direction, the magnetic flux from the coil is actively transmitted to the heat generating member, so that the temperature raising efficiency (heat generating efficiency) is improved. I understand. Therefore, it can be seen that it is effective to shorten the facing distance of the coil 25 in the entire width direction at the time of rising.
From the solid line W3 and the solid line W1, it can be seen that when the facing distance of the coil 25 to the heat generating member is varied according to the paper size, the heat generation efficiency in the non-sheet passing range is lowered and the edge temperature rise is suppressed. . Therefore, it can be understood that the temperature unevenness in the width direction of the heat generating member can be reduced even if the facing distance of the central portion is a condition of high heat generation efficiency which is advantageous at the time of rising.

  It should be noted that the present invention is not limited to the above-described embodiments, and within the scope of the technical idea of the present invention, the embodiments can be modified as appropriate in addition to those suggested in the embodiments. Is clear. In addition, the number, position, shape, and the like of the constituent members are not limited to the above embodiments, and can be set to a number, position, shape, and the like that are suitable for carrying out the present invention.

1 is an overall configuration diagram illustrating an image forming apparatus according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view illustrating a fixing device in the image forming apparatus of FIG. 1. FIG. 3 is a cross-sectional view showing a fixing belt in the fixing device of FIG. 2. FIG. 3 is a perspective view showing the vicinity of an induction heating unit in the fixing device of FIG. 2. It is a perspective view which shows another induction heating part. It is the schematic which shows the state where the opposing distance of an induction heating part is changing. It is the schematic which shows another state in which the opposing distance of an induction heating part is variable. It is the schematic which shows the state from which the opposing distance is variable in another induction heating part. It is sectional drawing which shows the state from which the facing distance of the induction heating part is changing. It is a block diagram which shows the principal part of the image forming apparatus in Embodiment 2 of this invention. It is sectional drawing which shows the fixing device in Embodiment 3 of this invention. It is sectional drawing which shows the fixing device in Embodiment 4 of this invention. It is the schematic which shows the experimental apparatus for an effect confirmation. It is a graph which shows the experimental result by the experimental apparatus of FIG. It is a graph which shows the experimental result following FIG. It is a graph which shows the result of another experiment for effect confirmation. It is a graph which shows the result of another experiment for effect confirmation.

Explanation of symbols

1 image forming apparatus body (apparatus body),
20 fixing device, 21 fixing auxiliary roller,
22 fixing belt (heating member, fixing member), 22a base material,
22b conductive layer (heat generation layer), 22c elastic layer, 22d release layer,
23 support roller (heating member), 24 induction heating part (magnetic field generating means),
25 coil, 28 heating roller (heating member, heating member),
30 pressure roller, 31 fixing roller (heating member, fixing member),
33 conductive layer (heating member), 34 nonmagnetic conductive layer,
38 thermistor, 40 high frequency power supply,
51 driving part (variable means), 55 holding member, 56 elastic member.

Claims (17)

A fixing device for fixing a toner image on a recording medium,
Magnetic field generating means for generating an alternating magnetic field;
A heating member that is disposed so that the front and back surfaces are sandwiched and spaced apart by the magnetic field generating means, and that generates heat by the alternating magnetic field;
Variable means for changing the facing distance of the magnetic field generating means to at least one of the front and back surfaces of the heat generating member according to the position in the width direction;
A fixing device comprising: 2. The variable means according to claim 1, wherein the facing distance within the range is controlled to be shorter than the facing distance outside the range according to the size of the recording medium in the width direction. Fixing device. When the size of the recording medium in the width direction of the recording medium changes from a small one to a large one, the opposing distance within the range in the width direction of the small one is smaller than the range in the width direction of the small one. The fixing device according to claim 1, wherein the fixing device is controlled to be longer than the distance. The fixing device according to claim 1, wherein the variable unit is controlled so that the facing distance is shortened over the entire width direction when the apparatus is started up. 5. The variable means according to claim 1, wherein only the facing distance with respect to a surface which is a heat generating main surface of the front and back surfaces of the heat generating member can be varied depending on a position in a width direction. The fixing device according to any one of the above. 6. The fixing device according to claim 1, wherein the variable unit deforms the magnetic field generating unit linearly or / and curvedly. The said magnetic field generation | occurrence | production means is a coil wound apart so that the front and back surfaces of the said heat generating member may be pinched | interposed once or several times, The said any one of Claims 1-6 characterized by the above-mentioned. Fixing device. The fixing device according to claim 1, wherein the heat generating member includes a conductive layer formed to have a Curie point of 350 ° C. or lower. 9. The fixing device according to claim 1, wherein a frequency of an alternating current supplied to the magnetic field generation unit in order to generate the alternating magnetic field is in a range of 10 k to 1 MHz. The fixing device according to claim 1, wherein the heat generating member is a fixing member that melts a toner image. The fixing member is a fixing roller that contacts a pressure roller that presses a recording medium to be conveyed,
The fixing device according to claim 10, wherein the magnetic field generation unit is disposed to face an outer peripheral surface and an inner peripheral surface of the fixing roller. The fixing member is a fixing belt stretched around a circumference,
The fixing device according to claim 10, wherein the magnetic field generation unit is disposed to face an outer peripheral surface and an inner peripheral surface of the fixing belt. The fixing belt is stretched between a support roller and a fixing auxiliary roller,
The fixing device according to claim 12, wherein the fixing auxiliary roller is disposed so as to come into contact with a pressure roller that presses a conveyed recording medium via the fixing belt. The fixing device according to claim 13, wherein the magnetic field generation unit is disposed to face an inner peripheral surface of the fixing belt with the support roller interposed therebetween. The fixing device according to claim 1, wherein the heat generating member is a heating member that heats a fixing member that melts a toner image. The fixing member is a fixing belt,
The heating member is a support roller that stretches the fixing belt together with a fixing auxiliary roller,
The magnetic field generating means is arranged to face the outer peripheral surface of the fixing belt and to face the inner peripheral surface of the fixing belt via the support roller,
The fixing device according to claim 15, wherein the fixing auxiliary roller is disposed so as to come into contact with a pressure roller that presses a conveyed recording medium via the fixing belt. An image forming apparatus comprising the fixing device according to claim 1.

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